The electron microscope is configured to obtain a magnified image of a face to be observed of a specimen under observation, by arranging the specimen inside an electron-optical lens-barrel, irradiating the specimen with a primary electron beam which is emitted from an electron gun and focused by magnetic lenses, and detecting secondary charged particles which are emitted from the specimen by the irradiation with the primary electron beam. (Hereinafter, the specimen under observation may be simply referred to as “the specimen”, the face to be observed is referred to as “the observation face”, and the magnified image is referred to as “the specimen image”.)
The electron microscope is configured to use a specimen holder, which is provided for observation of a cross-sectional specimen when the cross-sectional specimen is observed as the specimen under observation. The “cross-sectional specimen” means a specimen under observation having a cleaved portion. For example, the semiconductor manufacturers are performing analyses of defects in semiconductor wafers, manufacturing processes, and others by observing cross-sectional portions of cleaved semiconductor wafers by electron microscopes. In such cases, the cleaved semiconductor wafers and the like are the “cross-sectional specimen”.
The specimen holder for observation of a cross-sectional specimen is a member for facilitating fitting the cross-sectional specimen to the electron microscope. (See, for example, Patent Literature 1.) When the specimen holder, inside which the cross-sectional specimen is loaded, is inserted inside the electron-optical lens-barrel of the electron microscope, the specimen holder disposes the cross-sectional specimen at a predetermined location which is appropriate for observation. The specimen holder is configured to hold the cross-sectional specimen at the above location in such a manner that the observation face of the cross-sectional specimen perpendicularly intersects with the primary electron beam.
In particular, the specimen holder disclosed in Patent Literature 1 is configured such that the observation position of the cross-sectional specimen relative to the primary electron beam is unchanged at all times, and the cross-sectional specimen can be easily fitted to the electron microscope, so that it is possible to prevent the object from going out of sight. The conventional specimen holders, which are typified by the specimen holder of Patent Literature 1, are arranged such that a space exists around the observation face of the cross-sectional specimen when the cross-sectional specimen is loaded.
The scanning electron microscope is a type of the electron microscope. The scanning electron microscope is configured to have, in addition to the aforementioned configuration of the electron microscope, a function of scanning over the specimen with the primary electron beam by use of a magnetic-field or electric-field deflector which is arranged above an objective lens.
In the conventional scanning electron microscopes, the specimen is grounded when the specimen image is obtained. However, recently, use of the retarding (deceleration) method, in which a negative voltage is applied to the specimen when the specimen image is obtained, has been becoming predominant. The retarding method is a technique for obtaining a specimen image by applying a negative voltage (retarding voltage) of approximately hundreds of volts to several kilovolts to the specimen so as to decelerate the primary electron beam immediately in front of the specimen.
According to the retarding method, the voltage (acceleration voltage) Vacc applied to the primary electron beam accelerated by the electron gun and the voltage (retarding voltage) Vr applied to the specimen give the irradiation voltage Vi (which may be referred to as the landing energy) as follows.Vi=Vacc−Vr 
The scanning electron microscopes can obtain a specimen image at the same irradiation voltage with higher image quality when the retarding method is used than when the retarding method is not used (i.e., when the specimen is grounded).
For example, in either of the case where Vacc=1 kV and Vr=0.5 kV and the case where Vacc=0.5 kV and Vr=0 V, the irradiation voltage is the identical value of 0.5 kV. However, the resolution (i.e., the degree of clearness in the appearance of the specimen image) is better in the former case.
In addition, when a retarding means is used, the scanning electron microscope can also obtain a specimen image at an irradiation voltage (e.g., Vi=0.1 kV) lower than the minimum acceleration voltage (e.g., Vacc=0.5 kV) which can be realized by the electron gun.
As mentioned above, when a retarding means is used, the scanning electron microscope can realize high-resolution morphological observation of the observation face of the specimen. In addition, when a retarding means is used, the scanning electron microscope can achieve various effects including suppression of electrification of the specimen and reduction of damage on the specimen.
Further, the scanning electron microscopes can be classified into three types, the out-lens type, the in-lens type, and the semi-in-lens type. The out-lens type scanning electron microscope is configured such that the specimen is arranged at a position completely apart from the lens magnetic field of the objective lens. The in-lens type scanning electron microscope is configured such that the specimen is arranged in the lens magnetic field of the objective lens. The semi-in-lens type scanning electron microscope is intermediate between the out-lens type and the in-lens type, and configured such that the specimen is arranged at a position to which a portion of the lens magnetic field of the objective lens leaks.
Among the three types of scanning electron microscopes, the in-lens type scanning electron microscope is more advantageous than the other types because the in-lens type scanning electron microscope can most efficiently utilize the lens power of the objective lens and obtain a specimen image with high resolution. (Hereinafter, the in-lens type scanning electron microscope is referred to as the “in-lens SEM”.
However, in the in-lens SEM, the specimen is required to be arranged in the lens magnetic field of the objective lens. Therefore, the in-lens SEM is configured such that the specimen is loaded in the tip end of a dedicated specimen holder, and the specimen holder is inserted in the lens magnetic field of the objective lens. In the in-lens SEM having such a configuration, the specimen and the specimen holder are arranged in a narrow space.
On the other hand, according to the aforementioned retarding method, a negative voltage at the same level (on the same order) as the acceleration voltage of the primary electron beam is applied to the specimen. Therefore, in consideration of the possibility of electric discharge or leakage, almost no conventional in-lens SEM adopts the retarding method. Nevertheless, since a technique for preventing the electric discharge or leakage has been developed in recent years, the retarding method has been becoming able to be adopted in the in-lens SEM.